Because it determines the "reserve" of the pacemaker's output circuit, threshold measurement remains the single most important factor that will ultimately ensure successful pacing. In this review, we describe the various techniques and significance of threshold measurement in relation to the property of pacemaker output circuits. There are two basic types of pacing circuits: (1) those with constant current, as in most external (temporary) pulse generators, and (2) those with constant voltage, as in many implantable pulse generators. Current-limited pulse generators have features of both constant-current and constant-voltage circuitry. The current threshold is helpful in determining the integrity of the electrode-tissue interface and reflects the density of the current, which is the prime factor responsible for successful stimulation. Voltage thresholds are useful for information on lead position and integrity, especially when voltage and current are measured simultaneously. Impedance (calculated from voltage and current during stimulation) can be helpful in the diagnosis of lead fractures, insulation breaks, and position problems. Threshold and impedance are entirely unrelated factors, each providing specific and different information about a pacing system. Threshold may also be measured in terms of charge and energy and in relation to the width of the pulse (at a constant impulse amplitude). The concept of safety margins is important when measuring long-term threshold at the time of replacement of a pulse generator. We have analyzed this problem and have attempted to make acceptable recommendations in the absence of clear information in the literature about this subject.